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If you’re making a modern computer chip you’re going to need to put layer after layer of traces down. You’re also going to need layer after layer of insulator between your metal in order to not short circuit everything. But if you start growing oxides and depositing metals and such and so forth your wafer is going to end up wrinklier than yer grandma’s keister. That’s going to cause problems. Your layers won’t have uniform thicknesses anymore. Particularly the photoresist which means your chances at making a decent pattern degrade. Assuming you don’t end up with fatal defects from underdeveloping at the very least you lose feature size precision. Oh, and you end up with the occasional smug columnist making you visualize yer grandma’s keister. Heh.
Surely You Wouldn’t Be Bringing This Up if You Didn’t Have a Handy Gadget Solution?
Well you’re in luck Mr. Douglas. What you need is a Chemical Mechanical Planaraization rig. Or Chemical Mechanical Polishing. CMP. Whatever. (It makes a difference, but people often use the terms interchangeably.) Now, you could just deposit some borophosphosilicate glass on there and heat it up, expect the glass reflow, cover that topography like the snows o’ winter covering a shallow grave. In the olden days that was good enough; got the job done. These days the devices are smaller, the lines sharper. If you want real flatness, if you need to get down to a fifty angstrom step height over the surface of the entirety of your wafer, you want the real McCoy. CMP; accept no substitutes.